XMLString: Don't call catString if relativePath is null

https://xerces.apache.org/xerces-c/secadv/CVE-2017-12627.txt

Upstream status: svn revision 1819998

Signed-off-by: Baruch Siach <baruch@tkos.co.il>

--- trunk/src/xercesc/util/PlatformUtils.cpp	2018/01/03 18:58:30	1819997
+++ trunk/src/xercesc/util/PlatformUtils.cpp	2018/01/03 18:59:30	1819998
@@ -920,7 +920,10 @@
 
     XMLString::subString(tmpBuf, basePath, 0, (basePtr - basePath + 1), manager);
     tmpBuf[basePtr - basePath + 1] = 0;
-    XMLString::catString(tmpBuf, relativePath);
+    if (relativePath)
+    {
+        XMLString::catString(tmpBuf, relativePath);
+    }
 
     removeDotSlash(tmpBuf, manager);
 
